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					Recycling citrus waste
into animal feed
More than two million tonnes of pulp, peel and      Feed solution for solid waste
rag remain each year after citrus fruits are        From the waste peel, mare and seed from
processed into juice, frozen concentrate and        processing lines, researchers developed dried
sections. In the past, solid waste, the culls and   pulp, which is used extensively for feeding
surplus fruit, were dumped on wastelands or         dairy and beef cattle and is also suitable for
used as soil conditioners on cultivated land,       feeding other animals.
while liquid waste was ponded or flushed into
streams, lakes or sewers. However, all such         “Although it must be supplemented with
solutions have since proven unsatisfactory
                                                      some other feed, it contains significant
and dangerous, since a pile of rotting orange
peels soon begins to smell, underground             amounts of the protein, fats and minerals”
water supplies become contaminated and
the increased biochemical oxygen demand                 It contains about 8% moisture, 6% ash, 6%
kills aquatic life or exceeds the capacities of     crude protein based on total nitrogen, 6% crude
sewage-treatment plants.                            fat, 14% crude fibre and 66% nitrogen-free
                                                    extract. Although it must be supplemented with
Following thorough investigation of the             some other feed, it contains significant amounts
mounting problem by industrial and research         of the protein, fats and minerals required
organisations, several economically viable          in animal feed. About one tonne of feed is
solutions in the form of secondary products         obtained from 10 tonnes of cannery waste.
have arisen from their efforts. Now, 80 to              To make dried pulp, fresh peel is first
90% of citrus waste is converted into usable        ground in a hammer mill, to which 0,5 to 1%
products, such as dried pulp, molasses,             of lime is immediately added. Lime neutralises
pectin, essential oils, brined peel, citric acid,   the acids and catalyses the de-esterification of
limonene, feed yeast and biologically active        the pectin in the peel to form calcium pectate,
materials.                                          which facilitates pressing and drying. The

22   | MILK & JUICE MARCH ’11
                                                JUICE


amount is carefully measured in order to get            Citrus molasses is most commonly
the best pressing characteristics. Formerly,        extracted through a process of multiple-effect
the peel was allowed to stand in bins for           evaporation, with the first stage completed
about 45 minutes before pressing or drying          under positive pressure and the last stage
to allow time for the lime to react. Now the        under negative pressure. Intermediate stages
reaction time is shortened to 15 minutes by         may be used to increase the number of
continuously stirring the peel while it moves       kilograms of water evaporated per kilograms
slowly through a pug mill. Continuous presses       of fuel consumed. The evaporators used for
then remove as much liquid as possible, and         the purpose of producing citrus molasses
in some processing plants the pulp is heated        may be constructed of mild steel, although
by direct steam injection to about 50°C to          corrosion is something of a problem if not all
facilitate the pressing. The weight of liquid       of the acid is neutralised by the lime.
removed is about equal to the weight of the
pressed pulp.
    Direct-fired or steam-heated rotary kiln
                                                         “Citrus molasses is most commonly
driers are then used to remove the remaining         extracted through a process of multiple-
moisture from the pulp. In some mills, the            effect evaporation, with the first stage
pulp is given a preliminary drying in direct-        completed under positive pressure and
                                                    the last stage under negative pressure”
fired units and finished in steam-heated units.
Careful control of the drying rates and
temperatures is necessary to produce the
fluffy, light-coloured feeds that are considered         A more serious problem is scaling of the
desirable.                                          heat-exchanger tubes in the evaporators,
    Over the past 10 years, production of           because the scale builds up rapidly and
dried peel has increased to approximately           interferes with heat transfer and circulation,
200 000 tonnes per year.                            and must be removed about twice a week
                                                    by boiling with lye. The exact nature of the
Processing liquid waste                             scale has not been determined, but it is most
Press or drain liquor from citrus peel contains     probably composed largely of calcium and
5 to 7% sugar and a total of 10 to 12%              fibrous material.
soluble solids. It cannot be flushed into sewers         Another use for press juice is in the
or ponded, unless care is taken to reduce the       production of yeast, particularly Torulopsis
biochemical oxygen demand or to prevent             utilis, which grows rapidly and is therefore
bad smells. As a result, most of the press liquor   less susceptible to contamination than other
is concentrated to produce molasses.                yeasts. It is rich in vitamin B and is a good
    Citrus molasses is dark brown, bitter and       supplement in feeds.
contains about 57% total digestible nutrients.           About half the dried yeast is crude protein,
It is used mainly in cattle feeds, generally        but it is deficient in methionine, one of
mixed with other materials, although it can be      the essential amino acids, which is present
fed full strength. Some is mixed with wet citrus    in cereal proteins. A ration containing this
pulp and then dried to make a feed. It can          yeast with some cereal would provide all
also be used as a fermentation substrate in the     the essential amino acids required in animal
production of alcohol.                              feed. M&J

                                                                 MILK & JUICE MARCH ’11        | 23

				
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